Polymers and copolymers crosslinked with organic peroxides are known to have superior properties, particularly compared to polymers crosslinked by sulfur cure. These properties include high heat aging resistance, low percent compression set, decreased staining of metal, and easy production of colored products with enhanced color stability. In view of these beneficial properties, peroxide cure has a great deal of practical importance. A possible drawback of peroxide cure is that air must be excluded from the surface of a material during cure; if the air is not excluded, a tacky surface may result, due to cure inhibition by oxygen.
When oxygen comes into contact with an elastomer being crosslinked by an organic peroxide, the crosslinking reaction at the elastomer surface may be inhibited, or may not take place at all. Thus, the elastomer surface remains uncured. Therefore, curing rubber articles with peroxides is typically conducted in steam tubes, steam autoclaves, and air-evacuated closed molds designed to apply heat to the elastomer while excluding atmospheric oxygen during the crosslinking process.
Unfortunately, excluding air from these commercial processes involves considerable time and expense. In contrast, sulfur vulcanization of elastomers can be conducted using lower cost hot air ovens or tubes in which hot atmospheric oxygen poses no issue. While the sulfur curatives are generally lower in cost than organic peroxides, the types of elastomers suitable for sulfur cure are limited to unsaturated elastomers, e.g., poly(ethylene propylene diene), poly(butadiene), natural rubber, synthetic poly(isoprene), poly(styrene-butadiene) rubber, poly(butadiene-acrylonitrile) rubber and the like.
In many cases, manufacturers would like to switch from sulfur to peroxide cure using existing hot air ovens; however, curing with conventional peroxide systems under these circumstances would not be commercially viable due to the surface cure inhibition by oxygen. Various methods have been suggested for preventing surface cure inhibition by oxygen during free radical crosslinking. These methods have generally met with little acceptance.
U.S. Pat. No. 6,747,099 is directed to elastomer compositions that include bis-, tri- or higher polymaleimides and/or bis-, tri- or higher polycitraconimides.
U.S. Pat. No. 4,983,685 is directed to elastomer compositions that include at least 2.5 to 20 phr (parts per hundred rubber) of benzothiazyl disulfide.
U.S. Pat. No. 6,775,848 is directed to pore-free rubber articles prepared by dip-molding.
U.S. Pat. No. 4,376,184 is directed to rubber compositions that include an organopolysiloxane gum.
EP 0246745 is directed to elastomer compositions that include low molecular weight polymers of 1,000 to 15,000 as an additive.
U.S. Pat. No. 5,219,904 is directed to fluorine-containing elastomers that contain iodine and bromine.
U.S. Publication No. 2013/0131221 is directed to elastomer compositions that include at least one cellulose ester.
Generally, none of the previously described systems adequately provide a tack-free surface while concurrently providing desirable physical properties like superior compression. Moreover, previous known methods involving sulfur and peroxide cure typically are limited to unsaturated elastomers.
Thus, it is desirable to have organic peroxide formulations and methods which cure commercially available crosslinkable elastomers and polymers, both saturated and unsaturated, in the full or partial presence of atmospheric oxygen.